Retention and drainage aid for alkaline fine papermaking process

Abstract

A process in which fine paper is made by forming an aqueous cellulosic suspension comprising fibers, a precipitated calcium carbonate filler and a cationic starch strengthening agent, passing the suspension through one or more shear stages, draining the suspension to form a sheet and drying the sheet. The retention and drainage properties of the suspension are substantially improved via the addition of a cationic coagulant having a molecular weight in the range between about 2,000 to about 500,000 to the suspension prior to any of the shear stages, an anionic flocculant having a molecular weight of at least 500,000 to the suspension after the low molecular weight coagulant but before any of the shear stages, and an inorganic material selected from the group consisting of: bentonite, colloidal silica and other inorganic microparticle materials, to the suspension after at least one of the shear stages.

What is claimed is: 
     
       1. A process in which fine paper is made by forming an aqueous cellulosic suspension having a pH in the range between about 6.8 to about 9.0, said cellulosic suspension comprising fibers, a precipitated calcium carbonate filler and a cationic starch strengthening agent, passing the suspension through one or more shear stages, draining the suspension to form a sheet and drying the sheet, wherein a cationic coagulant having a molecular weight in the range between about 2,000 to about 500,000 is added to said suspension in an amount between about 0.001% to about 0.5%, based on the dry weight of said suspension, prior to any of said shear stages, an anionic flocculant having a molecular weight of at least 5000,000 is added to said suspension in an amount between about 0.001% to about 0.8%, based on the dry weight of said suspension, after said coagulant but before any of said shear stages, and an inorganic material selected from the group consisting of: bentonite and colloidal silica, is added to said suspension after at least one of said shear stages. 
     
     
       2. The process according to claim 1 wherein said coagulant has a molecular weight in the range between about 10,000 to about 500,000. 
     
     
       3. The process according to claim 1 wherein said coagulant is added to a thick stock of said cellulosic suspension and said anionic flocculant is added to a thin stock of said cellulosic suspension, said thin stock is a dilute aqueous suspension of said thick stock. 
     
     
       4. The process according to claim 1 wherein said coagulant is cationic and selected from the group consisting of: polyethylene imine, polyamines, polycyandiamide formaldehyde polymers, amphoteric polymers, diallyl dimethyl ammonium chloride polymers, diallylaminoalkyl (meth) acrylate polymers, and dialkylaminoalkyl (meth) acrylamide polymers, a copolymer of acrylamide and diallyl dimethyl ammonium chloride, a copolymer of acrylamide and diallyaminoalkyl (meth) acrylates, a copolymer of acrylamide and dialkylaminoalkyl (meth) acrylamides, and a polymer of dimethylamine and epichlorohydrin. 
     
     
       5. The process according to claim 1 wherein said anionic flocculant has a degree of anionic substitution of at least 0.01. 
     
     
       6. The process according to claim 1 wherein said high molecular weight anionic flocculants are selected from the group consisting of: copolymers of acrylamide and acrylic acid, and copolymers of acrylamide and acrylamido-2-methyl propyl sulfonate. 
     
     
       7. The process according to claim 4 wherein said coagulant is a polymer of dimethylamine and epichlorohydrin having a molecular weight of about 50,000. 
     
     
       8. The process according to claim 6 wherein said anionic flocculant is an anionic copolymer of acrylamide and acrylic acid having 30 mole % of acrylic acid. 
     
     
       9. The process according to claim 1 wherein said inorganic material is bentonite which is added in an amount of from about 0.03 to about 1%, based on the dry weight of said suspension. 
     
     
       10. The process according to claim 1 wherein said pH of said cellulosic suspension is in the range between about 7.2 to about 9.0.